1. Field of the Invention
This invention relates to apparatus for melting and supplying thermoplastic materials to a dispensing system.
2. Description of the Prior Art
Thermoplastic materials or so-called "hot melt" materials have been used for many years for various purposes, including as adhesives in the manufacturing of products such as disposable diapers and in the manufacturing of packaging. Historically, the thermoplastic material was converted from a solid to a molten state in a tank having heated walls. The melted material was maintained in the molten state in the tank in sufficient volume to supply one or more applicators or dispensers. If the job or application required a substantial volume of hot melt material, a substantially large volume of material was required to be maintained in the molten or melted state, necessitating a long warm up or start up time for the apparatus, as well as prolonged exposure of at least some of the molten material to heat and/or to oxygen.
To avoid these problems, so-called grid-type hot melt supply systems were developed in which the solid thermoplastic material was stored in a hopper and melted upon the top of a heated grid located at the bottom of the hopper. The melted material then passed through holes in the grid into a relatively small holding reservoir from which the molten material was supplied by a pump beneath the reservoir to a dispenser. Grid-type supply systems have been capable of melting and supplying thermoplastic material at a very high rate, and the molten material is not maintained in a molten state for prolonged periods of time to char, oxidize, or otherwise degrade. A typical grid type hot melt supply system is disclosed in U.S. Pat. No. 3,946,645.
These grid-type hot melt supply systems have typically comprised a reservoir with a heated grid mounted on top of the reservoir. A hopper for receiving the solid thermoplastic material was mounted atop the heated grid. Mounted beneath the reservoir was at least one pump for pumping the molten thermoplastic material through one or more supply hoses which were connected to the pump. A flow passage was provided from the reservoir outlet to the pump inlet.
One problem with these hot melt supply systems has been that, in use, various extraneous objects or impurities could enter the hopper. Larger objects and impurities were prevented from passing through the openings in the grid, but smaller objects could pass through the grid and eventually reach the pump, resulting in periodic failure or clogging of the pump. If the pump failed or became clogged, it had to be removed and repaired or replaced.
To avoid draining the system of the thermoplastic material if the pump needed to be removed, a manually operated gate-type valve was sometimes located in the molten thermoplastic flow path between the reservoir outlet and the pump. An example of such a valve is shown in U.S. Pat. No. 4,666,066. This valve made it possible stop the flow of molten thermoplastic material to the pump whenever the pump needed to be removed, and the pump could then be removed without the necessity of first draining the entire reservoir of molten thermoplastic material. Another example of a flow shutoff valve that could be used to shut off the flow of material to the pump is shown in U.S. Pat. No. 4,667,850. Removal of the pump has still been difficult because the motor for driving the pump had to be disconnected from the pump in the course of removing the pump from the system, and to facilitate quick disconnection of the driving motor from the pump, U.S. Pat. No. 4,666,066 also discloses the provision of a motor mount to enable the pump to be more quickly removed from the system.
Although the presence of the gate-type valve and the quick disconnection of the pump motor make it easier to remove the pump to change or to repair the pump, servicing the pump should be performed as infrequently as possible, since it requires that the system be shutdown, and system shutdown should be avoided, since it allows the material in the hopper and the reservoir to solidify. After the material solidifies, it must thereafter be remelted, resulting in loss of system efficiency and capability. The removal and replacement of the pump still results in long downtimes of the supply system, and substantial quantities of wasted thermoplastic material.
Pump servicing can be reduced by providing a filter in the material flow path between the reservoir and the pump, but such a filter must be periodically removed for cleaning, and the flow path should be closed when the filter is removed to prevent the inadvertent leakage of molten thermoplastic material from the flow path.